The field of the invention is magnetic resonance angiography (xe2x80x9cMRAxe2x80x9d), and particularly, studies of the human vasculature using contrast agents which enhance the NMR signals.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography (xe2x80x9cDSAxe2x80x9d) have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. Images showing the circulation of blood in the arteries and veins of the kidneys and the carotid arteries and veins of the neck and head have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged.
One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such xe2x80x9cdynamic studiesxe2x80x9d enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Earlier images in the sequence may not have sufficient contrast in the suspect vasculature, and later images may become difficult to interpret as the contrast agent reaches veins and diffuses into surrounding tissues. Subtractive methods such as that disclosed in U.S. Pat. No. 4,204,225 entitled xe2x80x9cReal-Time Digital X-ray Subtraction Imagingxe2x80x9d may be used to significantly enhance the diagnostic usefulness of such images.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins, and after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. Each measurement is referred to in the art as a xe2x80x9cviewxe2x80x9d and the number of views determines the resolution of the image. The resulting set of received NMR signals, or views, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. The total scan time is determined in part by the number of measurement cycles, or views, that are acquired for an image, and therefore, scan time can be reduced at the expense of image resolution by reducing the number of acquired views.
MR angiography (MRA) has been an active area of research. Two basic techniques have been proposed and evaluated. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue as a means for differentiating the NMR signal amplitude. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. Flowing blood, which is moving through the excited region, is continually refreshed by spins experiencing fewer RF excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
MR methods have also been developed that encode motion into the phase of the acquired NMR signal as disclosed in U.S. Pat. No. Re. 32,701. These form the second class of MRA techniques, which are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same spin motion. Angiographic images are then obtained by forming either the phase difference or complex difference between the pair of velocity-encoded images. Phase contrast MRA techniques have been extended so that they are sensitive to velocity components in all three orthogonal directions.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. As described in U.S. Pat. No. 5,417,213 the trick with this contrast enhanced (CE) MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CE-MRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds.
The short separation time between arterial and venous enhancement dictates the use of acquisition sequences of either low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first pass CE-MRA methods thus impose an upper limit on either spatial or temporal resolution. An additional detrimental effect of rapid imaging of the first pass of the bolus of contrast is the spurious modulation of k-space data resulting from the shape of the bolus of contrast. Current CE-MRA exams are of immense clinical utility, but due to the temporal-spatial limitations, they still fall short of x-ray DSA, the current xe2x80x9cgold standardxe2x80x9d.
Ideally, angiograms should be acquired with techniques which allow longer scan times, after the first pass of the contrast bolus. For example, intravascular contrast agents can provide significant signal enhancement of the blood pool for over one hour. Images acquired after the first pass of the contrast agent in the so called xe2x80x9csteady statexe2x80x9d portion of the examination have the advantages of providing SNR and resolution increases limited only by patient motion. Even images acquired using currently available extravascular agents, which show only weak decay of plasma T1 shortening, may be improved by longer acquisitions acquired after the first pass of the bolus. Steady-state images permit excellent vessel delineation, although venous enhancement can severely limit arterial visualization reprojection images.
Clearly, an acquisition method which combines the excellent arterial-venous separation seen in first pass CE-MRA with the high resolution, high SNR images acquired in the steady-state is desirable. One approach is to acquire an angiogram without regard for the venous enhancement, and then remove the venous signal as a post-processing step. These venous removal techniques, which are referred to as vessel xe2x80x9csegmentationxe2x80x9d, have been attempted by several researchers. Current methods of vessel segmentation, however, have proven to be of limited usefulness partially due to the difficulty in determining which voxels are artery and which are vein based solely on a spatial/geometric analysis of the vessels, or an analysis of their signal intensities.
The present invention is an improved CE-MRA method which employs the NMR data acquired during the first pass of a bolus of contrast agent through the region of interest (xe2x80x9cROIxe2x80x9d) to identify arteries, veins and unenhanced background tissue, and to use that time resolved information to segment NMR image data subsequently acquired during the steady state portion of the examination. The method uses a time resolved series of images acquired during the first pass of the contrast bolus to calculate arterial and venous contrast enhancement reference curves. These curves are used to determine which voxels in an image contain arteries and which contain veins and those voxels which are unenhancing, background tissue or noise. The arterial/venous determination may then used to suppress venous and background signal in a high resolution arterial image.
It has been discovered that the most powerful discriminator of arterial and venous signal is temporal in nature. For an intravenous injection, the contrast agent will pass, as a bolus, through the imaged arterial vasculature prior to passage through the venous system. In addition, the shape of the bolus is distorted as it passes from the arteries to the veins, the venous contrast enhancement curve being xe2x80x9cflatterxe2x80x9d than the arterial enhancement curve. Signal-verses-time contrast enhancement reference curves are obtained from a region of interest (ROI) analysis of the time resolved images. The time course of these reference curves is compared to the time course of each voxel intensity in the same time resolved image series. The result of the comparison is to assign a value, or weight, to each voxel. The weights are proportional to how well the time course of the voxel intensities compares to the time course of the arterial and venous reference curves. These weights can be used to segment images acquired during the steady-state phase of the scan.
An object of the invention is to provide a high resolution high SNR image of the human vasculature. NMR data may be acquired during the steady-state phase of the scan from which high resolution and high SNR images may be reconstructed. Voxels in these images may be segmented, or weighted, such that background tissues are suppressed and either venous or arterial signals are suppressed to provide the desired clinical image.
Another object of the invention is to provide a high resolution time course series of contrast enhanced vascular images. Peripheral k-space data acquired during the steady-state phase is segmented, and the segmented data is combined with the central k-space data acquired during the time resolved phase of the scan. A high resolution series of time resolved images are reconstructed from the combined k-space data. Since the central k-space data acquired during the first pass of the contrast bolus is employed, the series of images depict the inflow of contrast agent, while the depiction of vessel edges is provided by the segmented peripheral k-space data.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.